1. Technical Field of the Invention
This invention relates to bearing protectors, specifically non-contacting labyrinth seal type and their use in rotating equipment, especially equipment which is used in powder applications such as centrifugal pumps and pillow blocks.
2. Description of the Prior Art
An example of a plant, which is often knee-deep in a powder substance around pieces of rotating equipment, is the cement industry. The powder lays adjacent to the rotating equipment bearing chamber, and over time, if the correct seal is not employed, the powder penetrates the bearing chamber leading to premature equipment failure.
Machine tool spindles are another example where the high speed rotation of the spindle leads to premature bearing seal failure, typically because of the use of lip seals, which contact the shaft during operation and quickly wear creating a contaminant entry point.
Rotating equipment is a common generic term for equipment which includes a bearing arrangement typically consisting of at least one bearing which is housed in a bearing cavity or chamber. The bearing is lubricated and sealed between a rotor to stator interface to prevent the ingress or egress of a fluid or solid to the bearing cavity, since such unwanted material results in a reduction of equipment life.
Bearing seals are often also referred to as bearing protectors or bearing isolators. However, the use of such seals extends well beyond the protection of a bearing in rotating equipment. Accordingly, while reference will be made below to bearing protectors, it should be understood that this term is used, as far as the invention is concerned, in connection with seals having such wider uses.
The purpose of a bearing protector is to prevent the ingress of fluid, solids and/or debris from entering a bearing chamber. Equally, bearing protectors are employed to prevent the egress of fluid or solids from a bearing chamber. Essentially, their purpose is to prevent the premature failure of the bearing.
Bearing protectors of the contacting type can be as basic as a lip seal or more complex as in the case of a face/mechanical seal. Reference is made to WO 04/005770, which defines a contacting bearing protector with an axially floating seal face against an axially static seal face.
Contacting bearing protectors often require lubrication at the counter-rotational surfaces. In high shaft speed applications such as machine tool spindles, excessive heat can be created in marginal/zero lubrication. Therefore, while their use in powder applications is clearly preferable, sometimes it is not feasible because of the lubrication requirements of the application.
Non-contacting bearing protectors can be of repeller or labyrinth configuration. Reference is made to WO 06/005950 which discloses a non-contacting bearing protector with a static shut off device.
In a non-contacting bearing protector, the rotating component typically has a complex outer profile which is located adjacent and in close radial and axial proximity to a complex inner profile of the stationary component. Together these complex profiles provide a tortuous path preventing the passage of the unwanted materials or fluids.
Unfortunately, in powder applications, the powder often penetrates the close rotating clearances of conventional labyrinth seal technology, leading to blockage and rotational seizure.
An air purge system can be used to overcome powder entry, but this has a limited effectiveness since, while some of the air acts to blow away the powder from the bearing seal entry point, air is also directed into the bearing chamber. The air is often unclean, and contains moisture and contaminant particles which are clearly undesirable inside the bearing chamber, defeating the whole purpose of using a bearing seal.
Several attempts have been made to provide a non-contacting bearing with an air purge facility but, without the possibility of contaminated air entering the bearing chamber.
Orlowski U.S. Pat. No. 6,062,568, discloses a labyrinth seal with an air inlet, the air travelling through a plurality of passages within the seal, before exhausting to atmosphere. Orlowski relies on an axial locking mechanism to minimise/restrict contaminated air from travelling into the bearing chamber.
Observations on the Orlowski seal include the following:                Injected air, typically at up to 10 psi (0.75 bar) acting on a substantial surface area, creates a considerable thrust force. The rotor in Orlowski has a significant pneumatic surface thrust area, which, when loaded with pressure, in practice acts axially to open the seal from within. This leads to accelerated wear and heat generation of the axial locking member. This in turn increases the axial clearances between the rotor and stator, permitting contaminated air to flow into the bearing chamber. The contaminants cause deterioration of the lubrication media leading to premature bearing failure.        The drain orifice, often positioned at the 6 o'clock on the stator, is directly in-line with the air purge outlet. This means that purging air, is encouraged to exit by the easiest route, the drain orifice, rather than fully purging contaminant located in the close radial clearances between the stator and rotor. Not only is this an ineffective use of the air purge but, the pressure of the air is significantly reduced where it is needed: at the rotor to stator interface. This allows entry of contaminants.        If the air purge is not used, or temporally switched off, as often occurs in plants, the large cavities inside the bearing seal quickly fill up with powder/contaminants which penetrate the radial and axial clearances between the rotor and stator. The considerable clog area of these cavities can lead to seizure of the rotor and premature failure of bearing seal.        
A bearing protector with an air purge system should preferably create a substantially zero pneumatic thrust acting to axially extend the seal. This not only eliminates any possibility of accelerated wear between counter rotational components, it also reduces heat build up within the seal. The European machinery directive ATEX deals with temperature limits of components in contact with the atmosphere, if explosive substances are being processed. A bearing seal which can operate at relatively low temperatures is preferred.
Plant air pressurise can vary considerably depending on equipment location and pipe restrictions from the air source. High-pressure air travelling through small outlet orifices could be dangerous and/or damaging to the equipment. Equally, low pressure air travelling through substantially large outlet orifices can be ineffective at removing contaminants. Pressure control is desirable.
Purging air through the rotor to stator interface is a desirable feature for the efficient and effective use of an external air purge system. It is desirable that the 6 o'clock drain hole within the bearing protector stator is out-of-line with the flow of air through the bearing seal.
There should desirably be a minimisation of the likelihood of rotor clogging and seizure, in periods where air is not employed. This is particularly relevant when processing powders which solidify with vapour.